Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 13-10-2017, 06:29   #1
Registered User

Join Date: Aug 2016
Location: Becalmed in the Med
Boat: Prout Snowgoose 35
Posts: 258
Anchor bridle/snubber

Catamarans tend to have long bridles, often 7m or more.

When paying out rode, do you count the length of the bridle as part of the scope say 4:1, of do you go 4:1 then attach the bridle and let out more rode?

I've been including the length of the bridle part of my scope and usually gone 4:1 or 5:1, but wondering whether that is correct as although the pull from the boat is much lower down than it would be without a bridle, it's pulling on less rode overall.

What do you do? And do your change your practise depending on the depth of the water?
__________________

__________________
mikedefieslife is offline   Reply With Quote
Old 13-10-2017, 07:00   #2
Registered User

Join Date: Apr 2007
Location: Jupiter FL
Boat: temporarily boatless...
Posts: 715
Re: Anchor bridle/snubber

Regardless of mono or multihull, it is the total length that matters, because it is the total, plus the height of freeboard and the depth of the water, that determines the angle of the rode and the holding efficiency. Pete
__________________

__________________
pete33458 is offline   Reply With Quote
Old 13-10-2017, 07:13   #3
Registered User

Join Date: Sep 2007
Location: Palm Beach Gardens, Fl
Boat: Gemini, 1993 #379 34' Shearwater
Posts: 249
Re: Anchor bridle/snubber

If the bridle is Attached to the waterline, I canít the scope as the depth of the water versus the last of the road. Itís a bridal is attached above the waterline I canít scope as the attachment point of the bridal versus the depth of the water. Attaching below the waterline effectively reduces the height of the attached point.
__________________
Capt. Stuart Bell
Ranger R-25
stu@shearwater-sailing.com
captstu is offline   Reply With Quote
Old 13-10-2017, 15:00   #4
Registered User
 
mark_morwood's Avatar

Join Date: Jan 2007
Location: Cruising (Atlantic -> Med -> Carib -> Pacific)
Boat: Vancouver 36, Hobie 33, Catana 48, now all with new owners
Posts: 203
Re: Anchor bridle/snubber

As has been already stated, the bridle is part of the rode and should be included. For a theoretical calculation of scope, there are two numbers that matter:
- the length of the rode system (including bridle) to the attachment point on the boat that takes the load (where your bridle attaches - if two, imagine a single point on center line)
- the vertical distance to the bottom from the attachment point (depth of the water + the height of the attachment point above the waterline)

Don't forget to include the height above the water in the calculation as that can be a significant factor, particularly in shallow water with a high freeboard boat.

This is all in theory. In practice we found that while cruising rode length is often constrained by other constraints such as swing room, but it is important to know how to calculate actual scope to make sure you know what your holding is likely to be. We learnt with time to be comfortable with widely different scopes/holding depending on location, weather forecasts and our plans (staying aboard? going ashore for the day? etc).
__________________
mark_morwood is online now   Reply With Quote
Old 14-10-2017, 09:34   #5
Registered User
 
tomfl's Avatar

Join Date: Apr 2012
Location: Florida
Boat: Seawind 1000xl
Posts: 1,946
Images: 10
Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
SNIP

What do you do? And do your change your practise depending on the depth of the water?
Another consideration is if you are using all chain or part chain and part line.

I can only speak for my boat but I tend to use a much shorter bridle than you indicate to reduce possible chafe due to where the cleats on my hull are. I also normally use all chain since I have something like 175 feet of chain on my primary. My take is you need more line out that chain. That being said I try to go at least 7:1 with all chain and often will let out more; but always in stages. I let out water depth plus a few feet; drift back, let out a little more and drift back; repeating maybe 3,4, or 5 times, and then back down; then let out even more chain. All of this is usually in 10 feet of water. When I am in deeper water things change a lot; but I always try and anchor in fairly shallow water.
__________________
tomfl is online now   Reply With Quote
Old 14-10-2017, 09:49   #6
Registered User

Join Date: Aug 2016
Location: Becalmed in the Med
Boat: Prout Snowgoose 35
Posts: 258
Re: Anchor bridle/snubber

What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
__________________
mikedefieslife is offline   Reply With Quote
Old 14-10-2017, 10:40   #7
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Living on dirt waiting for our new yacht to be built.
Boat: Half built Bestevaer.
Posts: 10,552
Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
If your anchor is in 5m and your freeboard is 1m and you let out 21m of chain, your scope is 21/6= 3.5:1, not 4:1.

If you let out 16m of chain and a 5m bridle (assuming this is length to the bow roller. The total length of the bridal on a cat will be greater due to the beam) then there is still 21m out and the scope is the same 3.5:1.

The total weight of chain will be similar in both cases, assuming the chain still leads to the bow roller.
__________________
noelex 77 is offline   Reply With Quote
Old 14-10-2017, 10:47   #8
Registered User

Join Date: Aug 2016
Location: Becalmed in the Med
Boat: Prout Snowgoose 35
Posts: 258
Re: Anchor bridle/snubber

Fair enough. I never multiple the freeboard it doesn't make any sense to me. It's always the same height, it doesn't change regardless of the depth of water, so all my calculations are usually done from water level.
__________________
mikedefieslife is offline   Reply With Quote
Old 14-10-2017, 10:55   #9
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Living on dirt waiting for our new yacht to be built.
Boat: Half built Bestevaer.
Posts: 10,552
Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
Fair enough. I never multiple the freeboard it doesn't make any sense to me.
The holding power of the anchor is related to the angle of the upward pull on the anchor. 1m of freeboard or 1m of extra depth has the same effect on the geometry.

So the freeboard should be added to the depth if you want to calculate the scope correctly.
__________________
noelex 77 is offline   Reply With Quote
Old 14-10-2017, 11:08   #10
Registered User

Join Date: Aug 2016
Location: Becalmed in the Med
Boat: Prout Snowgoose 35
Posts: 258
Re: Anchor bridle/snubber

Very true. I suppose that is usually covered in my 'let a bit extra' out which I always seem to do.
__________________
mikedefieslife is offline   Reply With Quote
Old 14-10-2017, 11:22   #11
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Living on dirt waiting for our new yacht to be built.
Boat: Half built Bestevaer.
Posts: 10,552
Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
Very true. I suppose that is usually covered in my 'let a bit extra' out which I always seem to do.
A bit extra never hurts .

Dont worry about the snubber length just look at the amount of chain out at the bow (assuming there is not an large loop of chain between the bow roller and the snubber conection) divide this by the depth of water where the anchor is located plus the freeboard. This ratio is the scope.

Some people add the maximium rise in tide but this is more correctly termed the minimum expected scope.
__________________
noelex 77 is offline   Reply With Quote
Old 14-10-2017, 11:24   #12
Senior Cruiser
 
maxingout's Avatar

Cruisers Forum Supporter

Join Date: Dec 2006
Location: Fort Pierce, Phoenix
Boat: Privilege 39 Catamaran, Exit Only
Posts: 2,596
Re: Anchor bridle/snubber

I use 200 feet of 3/8 inch high test chain when anchoring. One of the most important parts of our anchoring system is a lazy loop of chain that makes the pull of the chain more horizontal along the sea bed. It works really well for us, and we always use the lazy loop. We can anchor on shorter scope, and the additional weight of the lazy loop makes it harder for wind gusts to dislodge the anchor.

MAXING OUT - JOIN TEAM MAXING OUT AS THEY SAIL AROUND THE WORLD ON THEIR PRIVILEGE 39 CATAMARAN - EXIT ONLY
Attached Thumbnails
Click image for larger version

Name:	ANCHOR-SYSTEM-1200.jpg
Views:	43
Size:	210.6 KB
ID:	157717  
__________________
Dave -Sailing Vessel Exit Only

http://SailingUNI.com
http://maxingout.com
http://PositiveThinkingSailor.com
maxingout is offline   Reply With Quote
Old 14-10-2017, 11:40   #13
Moderator
 
noelex 77's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: Living on dirt waiting for our new yacht to be built.
Boat: Half built Bestevaer.
Posts: 10,552
Re: Anchor bridle/snubber

A "lazy loop" adds a bit of chain catenary and if some chain is dragging on the bottom it reduces sheering.

The drawback is if the snubber breaks, the boat will move back some distance, and develop considerable momentum, placing a high shock load on the anchor.

If using this technique (and it does have merits) I would advise using two independent snubbers. Cats using a bridle often rig this routinely. Two independent snubbers are not often used on a monohulls, but they are a good precaution in extreme conditions.
__________________
noelex 77 is offline   Reply With Quote
Old 16-10-2017, 03:51   #14
Registered User

Join Date: Dec 2015
Posts: 188
Re: Anchor bridle/snubber

Quote:
Originally Posted by mikedefieslife View Post
What I was getting at is, say you are in 5m of water and your freeboard is 1m.

You let out 21meter of chain. to get a 4:1 scope. All is well.

Now say you attach your 5m bridle to the chain at the 16m mark and then let out a little more chain so that it's the bridle taking the strain.

Now you've still got a 4:1 scope, your rode is still 21m but you've lost some of the catenary effect of the chain as all the chain past the 16m is doing nothing more than hanging off the bridle.
The catenary effect is always irrelevant when it's most needed, that means whenever there is any significant load on the rode.
I'll give you a numerical example of this:

~10 mm chain, having effective weight of 20 N/m when submerged.
Horizontal load on the rode 10 kN.
Horizontal distance from anchor to the attachment point 50 m.
Vertical distance from anchor to the attachment point 12.636 m.
Straight line distance from anchor to the attachment point 51.572 m.
Distance along the chain (catenary)from anchor to the attachment point 51.592 m.
Added distance due to catenary 0.020 m (2 cm or 20 mm or ~25/32 inch).
Change of angle of rode at the anchor ~2.75 degrees.

For those more interested of the mathematics than the results:
Equation of the rode Y = 500 m * COSH(X/500 m)
End point of rode at the anchor (X =150m Y = 522.669 m) and at the attachment point (X = 100 m Y = 510.033 m) Angle of rode at anchor from horizontal 16,937 degrees. Scope 4.083

If there is only 2 kN of load on the rode, the extension due to catenary would be 0.506 m instead of 0.02 m. That would allow some absorbing of impact loading due to waves and gusts, but still less than a proper snubber. Also the end of rode at anchor would be horizontal, while at the attachment 27.5 degrees from vertical, a significant difference, unlike the 10 kN loaded case of only 2.75 degrees of difference. The vertical distance in the 2 kN load case is 12.76 m instead of 12.64 m , but that doesn't have much effect on the results, just made calcs easier.
__________________

__________________
Just Another Sa is offline   Reply With Quote
Reply

Tags
anchor, bridle

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Attaching a Bridle or Snubber to Anchor Line Ocean Girl Multihull Sailboats 45 08-03-2017 12:36
Single Bridle off Anchor Roller vs Double Bridle with Chocks? sully75 Anchoring & Mooring 4 02-08-2014 16:14
How to Make a Snubber-Bridle Question 67Therapy Anchoring & Mooring 53 26-03-2013 11:36
Anchor Bridle vs Single Line Snubber BessLB Anchoring & Mooring 33 29-12-2009 07:07
Anchor Rode Bridle and Snubber Benny Anchoring & Mooring 17 08-09-2009 16:46



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 15:04.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.